Zoisite--Anorthite--Calcite Stability Relations in H2O--CO2 Fluids at 5000 Bars: An Experimental and SEM Study

The reaction 2 zoisite + CO2 = 3 anorthite + calcite + H2O has been reversed experimentally in cold-seal pressure vessels using natural phases and H2O–C02 fluids generated by water-silver oxalate mixtures. Equilibrium has been determined at 5000 50 bars, 599 9 °C and 0–075 ± 0–010 XCO2. Extrapolation using the MRK equation of Kerrick & Jacobs (1981) gives an equilibrium curve of negative T–X slope consistent with bracketing runs at 500, 550 and 650 °C. The curve agrees only with a new bracket of Nitsch (in Hoschek, 1980), and is at higher XCo2 than all other experimental determinations and at lower XCO2 than those calculated from the thermodynamic data of Helgeson et al. (1978). Discrepancies are attributed to differences in starting materials and small errors in the thermodynamic properties of the phases. Reaction direction and equilibrium have been determined by observing surface textures of run products by SEM. Growth and solution textures are non-equivalent, permitting unequivocal determination of reaction direction even where the extent of reaction is small, an advantage over conventional and insensitive XRD methods which measure bulk changes in the charge. Dissolution features of anorthite and zoisite are defect-related indicating control by surface reaction, whereas calcite dissolves by both surface reaction and diffusion controlled processes. Margarite forms in most runs below 585 °C. Textural features, its restriction to the margarite stability field and comparison with feldspar solubility data demonstrate it is an equilibrium phase formed by incongruent solubility of anorthite and zoisite in H2O-CO2 fluids. Quench phases formed from the solute are consequently silica-rich, with implications for metasomatic processes in feldspar–epidote–bearing rock and fluid systems. Absence of margarite from runs with anorthite, zoisite and calcite in the zoisite stability field is apparently due to the fast growth rate of zoisite. The full equilibrium assemblage is zoisite–anorthite–calcite–margarite at these temperatures, and the degeneracy of the model system is unobtainable in experiments, and presumably, in nature.